Pattern separation deficits may contribute to age-associated recognition impairments

Abstract

Normal aging is associated with impairments in stimulus recognition. In the current investigation, object recognition was tested in adult and aged rats with the standard spontaneous object recognition (SOR) task or two variants of this task. On the standard SOR task, adult rats showed an exploratory preference for the novel object over delays up to 24 h, whereas the aged rats only showed significant novelty discrimination at the 2-min delay. This age difference appeared to be because of the old rats behaving as if the novel object was familiar. To test this hypothesis directly, rats participated in a variant of the SOR task that allowed the exploration times between the object familiarization and the test phases to be compared, and this experiment confirmed that aged rats falsely "recognize" the novel object. A final control examined whether or not aged rats exhibited reduced motivation to explore objects. In this experiment, when the environmental context changed between familiarization and test, young and old rats failed to show an exploratory preference because both age groups spent more time exploring the familiar object. Together these findings support the view that age-related impairments in object recognition arise from old animals behaving as if novel objects are familiar, which is reminiscent of behavioral impairments in young rats with perirhinal cortical lesions. The current experiments thus suggest that alterations in the perirhinal cortex may be responsible for reducing aged animals' ability to distinguish new stimuli from ones that have been encountered previously.

Figure 1. Schematic of the standard SOR testing used in Experiment 1

The testing arena was 30 cm by 30 cm with 30 cm high walls and it was painted black. The floor had a grid painted on it to ensure that the object placement was consistent between the familiarization phase and the test phase and to help distinguish arena A from arena B (used in Experiment 3). The white cylinders indicate the location that the objects were placed in during the familiarization phase (top arena). The schematic white rat is shown in the orientation of when the rat is first placed in the arena. In the object familiarization phase (top arena), a rat is placed in the arena to explore duplicate copies of an object (A1 and A2). The rat is then moved from the arena for a variable delay (2 min, 15 min, 2 hr, or 24 hr). Following the delay, during the test phase (bottom arena), the animal is returned to the arena to explore a triplicate copy of the objects presented during the familiarization phase (A3) and a novel object (B1).

Figure 2. A schematic of the presentation of objects used in novel/repeat SOR task (Experiment 2)

Experiment 2 consisted of familiarization phase 1 followed by a two hr delay and then test phase 1. The next day, rats participated in a familiarization phase 2, which was again followed by a 2 hr delay and test phase 2. During the first familiarization phase, duplicate copies of an object (e.g., C1 and C2) were presented and the rat was allowed a total of 4 min of exploration in the open arena. After a 2 hour delay, the rat was returned to the same testing apparatus. For half of the young and the aged rats the same two objects from the familiarization phase were again placed in the testing arena in the same location (C1’ and C2’; repeat condition). For the other 5 young and 5 aged rats two identical novel objects (D1 and D2; novel condition) were placed in the arena in the same location as the objects from the familiarization phase. In both cases, the rats were given 4 min to explore. After the test phase, the rat was returned to the colony room for 24 hours before performing another object familiarization phase. During this second object familiarization phase, the rat was given 4 min to explore two identical novel objects that were distinct from any of the objects used the previous day (E1 and E2) and in a different position within the apparatus. After a 2 hour delay, the rats that had participated in the repeat condition on previous day were exposed to the two novel objects that they had not been presented with (D1 and D2; novel condition). Conversely, the 5 adult and the 5 aged rats that were given a novel presentation on the previous day were exposed to the same objects from the object familiarization phase (E1’ and E2’; repeat condition).

Figure 3. Schematic of the SOR task with context change (Experiment 3)

In the object familiarization phase, duplicate copies of an object (e.g., F1 and F2) were presented to the rat, which was then allowed a total of 4 min of exploration in the open arena. After the familiarization phase, and a 2 min or a 24 hour delay, the rat was relocated to a different room that contained arena B. During the test phase in arena B, the two objects were placed in similar locations relative to the walls of the apparatus as in the familiarization phase and the rat was allowed 4 min of exploration. During the test phase, one object (F3) was the third copy of the triplicate set of the objects used in the familiarization phase, and the other was a novel object (G1).

Figure 4. Morris swim task performance of young and aged rats

(A) The X-axis is the day of testing and the Y-axis is the mean corrected integrated path length (CIPL) score. Higher CIPL scores indicate longer path lengths to reach the escape platform. All rats completed 4 days of spatial trials (circles) in which the platform was hidden below the surface of the water. These spatial trials were followed by 2 days of visually-cued trials in which the platform was visible (triangles). During the spatial trials, the aged (purple) rats had significantly longer CIPL scores compared with the young (green) rats (F_[1,312] = 86.98, p < 0.001; ANOVA). The performance of the aged rats, however, benefited significantly more when the escape platform was visible compared to the young rats (F_[1,82] = 32.42, p < 0.001; repeated-measures ANOVA). Error bars represent +/−1 standard error of the mean. (B) The mean CIPL scores of individual young (green) and aged (purple) rats on Day 4 of spatial testing. The horizontal black lines indicate the mean CIPL for each age group. In the aged rats, CIPL score values below the grey horizontal line represent rats that performed within 1 standard deviation of the young animals. Only 7 aged rats met this criterion while the other 36 rats had CIPL scores that were at least 1 standard deviation above the young rat mean.

Figure 5. SOR task performance (Experiment 1)

(A) The mean discrimination ratio of the adult (black) and the aged rats (grey) measured during the test phase for the four different delay conditions. A higher discrimination ratio indicates that the animal spent more time exploring the novel object relative to the familiar object. Overall, the aged rats had significantly smaller discrimination ratios when compared to the adult rats (F_[7,128] = 7.26, p < 0.001; ANOVA). (B) The mean amount of time young (black) and aged (grey) rats spent exploring the familiar and the novel object during the test phase for the four different delay conditions. There was a significant main effect of novel versus familiar objects on exploration time (F_[1,128] = 122.03, p < 0.001; repeated-measures ANOVA). Additionally, the interaction effect of age group (adult versus aged), and object (novel versus familiar) on exploration time during the test phase was significant (F_[1,128] = 22.08, p < 0.001). Error bars represent +/−1 standard error of the mean.

Figure 6. Novel/repeat SOR task performance (Experiment 2)

The mean exploration time of the adult (black) and the aged rats (grey) measured during all four episodes of exploration. In the young rats, there was only a significant reduction in the total exploration time when the object presentation was the same between both the familiarization and the test phases (repeat condition; p < 0.02; Tukey HSD). In contrast, the aged rats showed a reduction in total exploration time between the familiarization and test phases both when the object presentation was repeated and when it was novel (p < 0.05; Tukey HSD). Error bars are +/−1 SEM.

Figure 7. SOR task with context change task performance (Experiment 3)

(A) The mean discrimination ratio of the young (black) and the aged rats (grey) measured during the test phase, in arena B, for the 2 min and 24 hour delay conditions. A higher discrimination ratio indicates that the animal spent more time exploring the novel object relative to the familiar object. Both the young and aged rats had a higher discrimination ratio after a 2 min delay compared to the 24 hour delay as indicated by the significant main effect of delay duration (F_[1,28] = 11.08, p < 0.005; repeated-measures ANOVA). For the 24 hour delay, neither the young nor the aged rats distinguished between the novel and familiar object as indicated by the lack of a significant interaction effect between age group and delay condition (F_[1,28] = 0.29, p = 0.60; repeated-measures ANOVA). (B) The mean amount of time young (black) and aged (grey) rats spent exploring the familiar and the novel object during the test phase for the 2 min and the 24 hour delay conditions. Unlike experiment 1, in which the context was congruent, when the context changed between the acquisition and the test phase, the adult and the aged rats spent significantly more time exploring the familiar objects for the 24 hr delay relative to the 2 min delay (p < 0.05; repeated contrasts). In contrast, the exploration time of the novel objects was not significantly different between delay conditions (p > 0.05 for all comparisons; Tukey HSD). Error bars represent +/−1 standard error of the mean.